This invention relates to the generation and control of small motions and displacements, both translational and rotational.
In precision instruments, especially optical instruments, it is often desirable to be able to adjust the position of the various elements of the instrument to be able to achieve the best possible performance, such as tuning a laser cavity to peak the power. Similarly, it is often desirable to accurately position a specimen for observation by a microscope, X-ray source or other instrument. This involves making very small adjustments in the instrument. In precision instruments, these motions may range from a fraction of a millimeter to a micrometer. In optical instruments, one is interested in making adjustments that are a small fraction of the wavelength of the light for which the optical system is designed, requiring motions with magnitudes from a micrometer to an angstrom.
These motions and adjustments are currently being made with Micrometers, compound screws and piezoelectric crystals. Micrometers are limited to the larger motions in the range of interest because the technology of making precision screw threads limits the thread pitch to about 1.60 threads to the millimeter. One turn of such a Micrometer produces about 625 micrometers of motion. If one can reliably produce a half-degree angular motion of the thimble of the Micrometer, then one may reliably produce a 0.87 micrometer motion. Use of optical encoders to read the angular position may improve accuracy some but the increased precision is soon overwhelmed by the backlash and other imperfections in the mated threads and supporting structure of the Micrometer. A precision Micrometer is limited to resolutions and motions larger than about 0.50 micrometer.
To achieve smaller resolutions, the Micrometer is often combined with some form of lever which further provides some reduction in the resulting motion from the Micrometer. Levers are limited in their ability to provide high reduction ratios in a small space. A lever that produces a 10,000:1 reduction requires its arm length to be in that same ratio. Features smaller than about 0.5 millimeter are difficult to manufacturer for the same reason as threads being limited to 1.6 per millimeter. If 0.5 millimeter is the smallest arm length, then a 10,000:1 reducer requires a long arm of 5 meters in length.
Compound screws offer much higher average mechanical advantages than single screws (e.g., Micrometers), but imperfections in the manufacture of the screw threads lead to cyclic run-out errors. The cyclic run-out errors in compound screw threads generate an oscillating axial component of motion superimposed upon the desired linear translation of the spindle. The oscillating axial motions make compound screw threads incapable of materially improving on the accuracy of the basic precision Micrometer.
Piezoelectric devices have been employed to generate small motions. These devices are quartz crystals which are exposed to an electrostatic fields across them. If the axes of the crystal lattice are appropriately oriented with respect to the electric field, then a small (but perceptible) change occurs in the dimensions of the crystal lattice. This results in a dimensional change for the whole quartz crystal. These devices can produce motions down to very small quantities (an angstrom or less) but suffer sizeable hysteresis effects (five to ten percent of full scale) which prevent the device from returning to the same point every time the voltage is adjusted to the apparently appropriate value. To overcome the hysteresis effects, piezoelectric devices are often fitted with optical encoders to measure their motion (position). The voltage is then adjusted to obtain the desired reading from the optical encoder. In these applications, the lower limit of accuracy (about 0.01 micrometer) is controlled by the accuracy with which optical encoders may be produced. The stability of these devices is further limited by thermal expansion of the material on which the optical encoder bars are deposited, usually quartz also.
The prior art which utilizes pivots is incapable of controlling translational or rotational motions of the small magnitude required for precision optical instruments. The current state of the art generally utilizes relatively large rigid levers and relatively small pivots or flat blade flexures, sometimes referred to as "film hinges." Such systems usually make no attempt to control the undesired translational or rotational components of motion. Said pivot systems usually have reduction ratios too large for use with precision instruments. Moreover, existing motion transducers utilizing pivot systems are susceptible to errors due to thermal expansion, hysteresis, and thermo-elastic effects which prevent accurate and repeatable motions.
Therefore, it is desirable in many applications to provide a motion transducer which produces accurate and repeatable small motions which are not susceptible to errors introduced by imperfect machining, thermal expansion, hysteresis or thermo-elastic effects.